The recovery of plastics from waste streams such as durable goods is a considerable challenge due the presence of several types of plastics as well as non-plastics contaminants. High purity flakes recovered from a recycling process can be extruded and pelletized to make a high quality product, but such extrusion can be difficult and the quality of the product can be poor unless certain types of contaminants are reduced to very low levels prior to extrusion.
Such problematic contaminants include, among others, non-plastics such as wood, particle board, paper, cardboard, rubber, textiles, metallic coatings, wires and circuit boards. Because such materials do not properly melt during the recycling process, they can quickly block melt filtration equipment used during the extrusion step even when these contaminant materials are present in small amounts. Non-melt particles smaller than the size of screens used for melt filtration can pass through the melt filtration equipment and end up as cosmetic and/or mechanical defects in products. In addition, cellulose-based contaminants (i.e., wood, particle board, cardboard and paper) can start degrading at extrusion temperatures typical for many thermoplastics. Therefore dark specks of degraded material can disintegrate from the original contaminant particles and pass through the melt filtration equipment. In addition, some plastics contain coatings, such as metallic paint or metal plating, that can cause similar difficulties in the extrusion step and in the products derived from extrusion of recycled plastics.
Some thermoplastics and thermosets found in durable goods are rigid foam materials known as structural foam. In some cases, this structural foam may contain flame retardants, glass fibers, residual blowing agents or other additives that could reduce the mechanical properties or marketability of the final product. In addition, the porous structure of these structural foams means that their density is lower than the density of the actual solid portion of the material.
In order to create flakes of a size suitable for extrusion from durable goods, the recycling process can include at least one and typically two or more size reduction steps. These steps can include the use of shredders, hammer mills, rotary grinders, granulators, or various other size reduction processes known in the art.
For at least some of the size reduction steps, the mixture containing plastics from durable goods also typically contains ferrous metals as well as slightly magnetic metals, such as alloys containing nickel, cobalt, molybdenum, vanadium, chromium or titanium. During the size reduction process, the high pressures and/or rapid cutter speeds tends to embed small fragments of these magnetic materials into some of the non-magnetic materials in the mixture. Small fragments worn from the size reduction equipment itself can also embed in these same non-magnetic materials.
Plastic recycling processes also typically include a wide array of conveying equipment, bins and chutes, many of which are made of steel. Over time, the steel abrades from equipment in contact with the plastics-rich mixture, and this abraded magnetic material can therefore be present in the mixture in trace amounts. Screw conveyors are one form of conveying equipment where this principle may occur, but there are other locations throughout the process as well.
Recycling processes can use high strength magnetic rods to prevent the accidental introduction of metals, such as small metal screws, into the extruder. Arrays of such magnets are, for example, sold by vendors such as Eriez Magnetics (Erie, Pa.) as grate magnets. Because there are very few such magnetic particles, the magnetic rods can easily capture the particles. The rods may be cleaned relatively infrequently and still perform their function of capturing the stray magnetic particles.
Another process sometimes included in the recycling process for durable goods is density separation. One method for separating plastics by density at densities greater than 1.0 is to use slurries of particulates, such as titanium oxide, magnetite or ferrosilicon. The particulate slurries can have a well-defined particle size distribution for improved separation.
In the following, additional methods, systems and devices are described for the selective removal of contaminant materials from plastic flakes.